Am. MidI. Nat. 160:386-399

Use and Selection of Bridges as Day Roosts by Rafinesque's Big-Eared Bats

FRANCES M. BENNETT' Department of Forestry and Natuml Resources, Institute of Environmental Toxicowgy, Clemson University,

Pendlef.on, South Carolina 29670

SUSAN C. LOEB2

USDA Forest Service, Southern &seaTch Station, Clemson, South CaTolina 29634

.lVlARY S. BUNCH South Carolina DepaTlmen! of Natural Resources, Rendlelon, 29670

AND

WILLIAM W. BOWERMAN Department of Forestry and Natural ResOllrc.es, Institute of Environmental Toxicclogy, Clemson University,

Pendleton, South Carolina 29670

ABsrnAcr.-Rafinesque's big-eared bats (Cc1)'llOrhinus mjinesquit) use bridges as day roosts in parts of their range, but information on bridge use across their range is lacking. From May to Aug. 2002 we surveyed 1129 bridges (12.5%) within all 46 counties of South Carolina to determine use and selection of bridges as day roosts by big-eared bats and to document their distribution across the state. During summer 2003, we visited 235 bridges in previously occupied areas of the state to evaluate shm-Herm fidelity to bridge roosts. We found colonies and solitary big-eared bats beneath 38 bridges in 2002 and 54 bridges in 2003. Construction type and size of bridges strongl}' influenced use in both years; bats selected large, concrete girder bridges and ayoided flat-bottomed slab bridges. The majority of occupied bridges (94.7%) were in the Upper mid Lower Coastal Plains, but a few bridges (5.3%) were located in the Piedmont. Rafinesque's big·eared bats were absent beneath bridges in the Blue Ridge l\'fountains. We established new records of occurrence for IO counties. In the Coastal Plains, big-eared bats exhibited a high degree of short-temlHdelity to roosts in highway bIidges. For bridges that were occupied at least once, mean frequency of use was 65,9%. Probability of finding bats under a bridge ranged from 0.46 to 0.73 depending on ",hethel- the bridge was occupied in the previous year. Thus, bridges should be inspected three to fiye times in a given year to determine whether they are being used. Regional bridge roost suryeys may be a good method for determining the distribution of C. rafinesquii, particularly in the Coastal Plains, and protection of suitable bridges may be a viable conservation strategy where natural roost sites are limited.

INTRODUCTION

Rafinesque's big-eared bat (C01)'JlO1llinus mfinesquit) is found in the southern and midwestern United States (Fig. 1) and is one of the least studied bats in North America (Hanrey et al., 1999). Despite having a relatively widespread distribution, this species is considered uncommon and is recognized as a species of special concern across most of its range (Hurst and Lacki, 1999; Martin et al., 2002). However, because Rafinesque's big-cared bats are not easily captured or detected with standard methods (e.g., mist nets, acoustic

I Present Address: Fish and Wildlife Branch, Saskatchewan Environment, 3211 Albert Street, Regina, SK S4S 5W6, Canada

2 Corresponding author: e-mail: sloeb@fs.fed.us; Phone: 864-656-4865

386

2008 BENNETT ET AL.: BAT RooSTS 387

3M 71 , , , 1

FIG. I.-Upper right: range ofRafinesquc's big-eared bats. Enlargement shows the four physiographic provinces of South Carolina and the locations of bridges used by Rafinesque's big-eared bats, late "May through mid-Aug. 2002-2003

sampling), it has been difficult to estimate their relative abundance and determine their geographic distribution.

Historical accounts, museum specimens and incidental capture records place this species in the Blue Ridge Mountains, the Upper Coastal Plain and the Lower Coastal Plain physiographic regions in North Carolina, South Carolina and Georgia (Handley, 1959; Clark, 1990; IvIenzel el al., 2003). In the Carolinas, Rafinesque's big-eared bat is commonly associated with bottomland hardwood forests (Clark, 1990) which are most abundant ",ithin the Upper and Lower Coastal Plains (Conner, 1993). Although bottomland hardwood forests also occur in the Piedmont, this bat appears to be absent from this physiographic region (Menzel et a.1., 2003). It is not clear whether the Piedmont truly does not support populations of big--eared bats or v· .. hethel' there have been insufficient sampling efforts in this region. A reliable lrtethod for locating Rafinesque's big-eared bats is clearly needed to determine their population status and distribution.

Rafinesque's big-eared bats are non-migratory and use tree ca\ities, caves, mines, buildings and other man-made structures for roosting (Barbour and Davis, 1969). Like most ca\ity-roosting species, Rafinesque's big-eared bats that use tree cmities and bridges frequently switch roost sites (Lance et al., 2001; Trousdale and Beckett, 2005), whereas cave roosting Rafinesque's big-eared bats rarely switch roosts (Hurst and Lacki, 1999). In the Coastal Plains, naturally occurring structures include cavities in large diameter gum (Nyssa. sp.) and cypress (Taxodium sp.) trees (Clark, 1990; Gooding and Langford, 2004; Trousdale and Beckett, 2005). Artificial sites are str:ucturally similar to natural ca\ities, and include dimly lit areas in abandoned buildings, cisterns, wells and highway bridges (Barbour and Da\is, 1969; Clark, 1990; Lance et 0.1., 2001; Mirowsky et 0.1., 2004; Trousdale and Beckett, 2002, 2004; Ferrara and Leberg, 2005a). Both artificial and natural structures are used as day and night roosts year-round, but frequency of use in anthropogenic structures peaks during "tvIay-Aug. when maternity colonies appear (Felts and \Vebster, 2003; Trousdale and Beckett, 2004). Thus, summer is the most appropriate time to conduct surveys in artificial structures, particularly bridges (Ferrara and Leberg, 2005b).

388 THE AMERICAN MIDLAND NATURALIST 160(2)

The relative use of artificial versus natural structures may depend upon the availability of structures in each physiographic region. Rafincsque's big-eared bats more commonly roost in artificial structures in the southern portion of their range, and in natural roost sites in the northern portion of their range (Jones. 1977). The Coastal Plain lacks an abundance of natural roost sites (i.e., large trees) because many were han'csted oyer a century ago. However, artificial structures now arc widespread and are frequently used as roost sites. By contrast, in the northern portions of the range natural roost sites such as tree cavities, rock houses, abandoned mines and caves are more frequently used (Bunch et ai., 1998; Hurst and Lacki, 1999L possibly because of their higher occurrence on the landscape.

Type of bridge constmction is the strongest predictor of bridge occupancy by Rafinesque's big-eared bats (Lance el al., 2001; McDonnell, 2001; Trousdale and Beckett, 2002). Bats roost in the space between girders on the underside of bridges and have not been observed in enclosed and concealed expansion joints (Ferrara and Leberg, 2005a). In South Carolina, three main bridge types are present: flat-bottomed slab bridges, multi-beam (MB) girder bridges and T-beam (TB) cast-in-place girder bridges (L. R. Floyd, South Carolina Department of Transportation, unpubl.). !vlB bridges are variable in structure, but generally consist of parallel beams that span the entire length of the bridge and sometimes are referred to as I-beam or channel beam bridges. TB bridges also have parallel beams that span the entire length of the bridge, but the support beams are intersected at right angles by cross beams. Although Rafinesque's big-eared bats most frequently use girder bridges in Louisiana and North CaroHna (Lance et ai" 2001; McDonnell, 2001), it is not known whether they select either of the two girder type bridges found in South Carolina.

Most studies of bridge use by Rafinesque's big-eared bats have been conducted within relatively small geographic areas restricted to the Coastal Plain regions (Lance el 0.1., 2001; McDonnell, 2001; Trousdale and Beckett, 2002; Felts and Webster, 2003). Limited bridge surveys have been conducted in South Carolina, but no day-roosting bats were found under the 44 bridges examined (Keeley and Tuttle, 1999). The objectives of our study were to: (1) document the use of bridges by Rafinesque's big-cared bats in South Carolina, (2) conduct a statewide bridge survey to determine their distribution across the state, (3) evaluate bridge attributes such as size and type that influence occupancy and (4) determine short term bridge fidelity and the number of visits needed to document presence, By identifying bridge types used by Rafinesque's big-eared bat and its fidelity to these structures, it may be possible to improve survey methods across its range.

METHODS

srUDYAREA

South Carolina consists of four physiographic regions: the Blue Ridge :Mountains, the Piedmont, the.Upper Coastal Plain and the Lower Coastal Plain (Fig. 1), The climate of South Carolina is warm temperate to subtropical and is characterized by short, mild \\~nters and long, hot and humid summers. Rainfall occurs throughout the year, but peak levels occur during the "\\~nter months in the mountains, and in :Mar. andJul. throughout the rest of the state. Average monthly rainfall amounts range from 11.4 em to 17.3 cm in the mountains, 7.4 cm to 11.7 em in the Piedmont and 6,0 cm to 16.6 cm in the Coastal Plains.

The Blue Ridge l\'Iountain region, a part of the southern Appalachian .Mountains, is situated in the upper northwestern portion of South Carolina, This region covers approximately 1.9% of the state, has a mountainous topography and ranges in elevation from 366 to 1067 m. Oak-hickory (Querclls sp, - Cm)'a sp,), oak-pine (Quercus sp. Pinus sp,)

2008 BENNE'll' IT AL,: BAT ROOSTS 389

and loblolly-shortleaf pine (P. taeda - P. echinata) arc the dominant forest types (Conner, 1993).

The Piedmont region is adjacent to the Blue Ridge 1.fountains and covers 31.9% of South Carolina. It has a rolling topography and ranges in elevation from 91 to 366 m. Urbanization and agriculture are common in this region; the dominant forests are loblolly­shortleaf pine forests. Localized stands of mixed pine-hardwoods and bottomland hardwood forests consisting of oak-bald cypress-tupelo gum (Quercus sp. - Taxodium distichwn - N)'ssa sp.) trees also are found in the Piedmont, but are concentrated in areas adjacent to the Upper Coastal Plain (Conner, 1993).

The Upper and Lower Coastal Plain provinces cover the largest area of South Carolina (66.2%), extending 193 to 241 km inland from the Atlantic Ocean. The topography of this region is flat; the highest elevation is 91 m. Forests in both Coastal Plain regions are dominated by lobloHy-shortleaf and longleaf-slash pine (P. paluslris - P. eUioUt) forests; however, bottomland hardwood forests are more extensive in these physiographic provinces than any other in the state (Conner, 1993). The Upper Coastal Plain has comparatively more urban, agriculture and .other non-forest cover types than the Lower- Coastal Plain region.

2002 STATEWIDE BRIDGE SURVEY

vVe conducted a county-by-·county survey from 22 ?-,'Iay to 8 Aug. 2002. Bridge data including shucture type, construction material, latitude/longitude, feature crossed (i.e., lvatenvay), unique identification number and bridge length and Vtridth were obtained from the South Caroliila Department of Transportation (SC DOT; L. R. Floyd) South Carolina Department of Transportation, unpubl.). For each of the 46 counties in South Carolina, we grouped and surveyed bridges according to type (slab, ME and TB). Because bats rarely roost in bridges over roadways and train tracks (Erickson, 2002) we only surveyed bridges over water bodies. \Ve surveyed bridges on public roads, including those on National Forests and National Vvildlife Refuges. For safety reasons, we did not survey bridges on interstate highways. Each bridge was surveyed once.

For the first 9 d of the survey, we used a stratified random sampling design based on bridge type and inspected bridges in proportion to their occurrence. Slab bridges were the most common bridge type over water in ~outh Carolina (n = 4025), followed by MBs (n = 1616) and TBs (n = 676). Based on the results of this initial sampling period (Bennett, 2004) and data from previous studies (Lance et a.1., 2001; McDonnell, 2001), we modified the study design to increase the likelihood of locating bats under bridges. For the remainder of the survey, we inspected bridges in the following order of precedence: TB, MB, and slab. vVe attempted to inspect every TB blidge over water and simultaneously surveyed a randomly generated subset of ~'IB and slab bridges.

\Ve inspected the underside of each bridge during the day for presence of bats with 1,000,000 candle-power flashlights. Data collected included: date, county, latitude and longitude, physiographic region, bridge type and material, number of Rafinesquc's big­eared bats present, number and species of other bats present, presence of bat feces and disturbance level. If bats were found under a bridge, we recorded details of the roost location and the group type (maternity colony or solitary). Where possible, independent counts of pups and adults were made by at least two field personnel and compared to ensure the most accurate count of bats. In some instances, total counts were not made to reduce disturbance to the bats. In these instances, we recorded an approximate range of the numbers of bats present. For data analysis, we used the lowest estimate.

390 THE AMERICAN MIDLAND NATURALIST 160(2)

"\Ve rated disturbance beneath each bridge on a discrete scale of 0-3. Bridges with no obvious disturbances were given a rating 0[0, low levels of disturbance were recorded as 1, medium levels of disturbance were recorded as 2, and bridges with high levels of disturbance were given a rating of 3. Disturbance factors included presence or evidence of humans such as trash, vandalism, footprints, all tClTain vehicle tracks and heavy vehicular traffic on the sUlface of the structure.

2003 BRIDGE SURVE\S AND ROOST MONITORING

Vve conducted bridge surveys from 23 :May to 1 Aug. 2003 using the same methods as in 2002. Although the 2003 field sun'cy \vas similar in execution to the 2002 statewide survey, there were two important differences. First, we did not survey the entire state. Instead, we focused surveys in areas where big-eared bats were found in 2002. Second, we inspected bridges occupied in 2002 several times in 2003; most bridges with big-eared bats "were surveyed every 2-3 wk so that bridges were examined up to five times. 'Ve also inspected additional bridges over water that were not visited in 2002, but were 'within occupied areas of the state. If a bridge was occupied in 2003 b~t not in 2002, we also monitored it regularly. However, due to restricted access, some bridges «10) were only inspected once in 2003.

DATA ANAL\SIS

'Ve used likelihood ratio chi-square tests (PROC FREQ; SAS, 2002) to determine the association bet,,·:een the presence of big-cared bats and qualitative attributes of bridges (type, physiographic region and disturbance) in 2002 and 2003. Due to small sample size, we used a Fisher's exact test to evaluate the association between the presence of big-eared bats and the occurrence of other bat species in 2002, and big-eared bat presence and disturbance in 2003 (Freeman and Halton, 1951). Associations between the presence of bats and quantitative attributes of bridges (length, width and area) were assessed using the KruskalgVallis one-"way analysis of variance. ,"Ve were unable to attain bridge size for 11 bridges in 2002 and eight bridges in 2003. Because of differences in sampling procedures between years, we analyzed data from 2002 and 2003 separately. Furthermore, because our sampling procedure was biased against slab bridges and no bats ,,·:ere found under these structures, we also ran the above analyses after excluding slab bridges from the dataset. '!.ole were unable to determine the association between bridge mateIial (concrete, timber, steel alloy) and presence of bats because material and bridge type were not independent. .

'iVe used logistic regression analysis with a stepwise selection process (0: = 0.05) to determine bridge attributes selected or avoided by Rafinesque's big-eared bats (PROC LOGISTIC). 'Ve determined the goodness of fit of the logistic regression equations for binary response models (Hosmer and Lemeshow, 2000). Models were run ".:ith and without slab bridges. Because bridge area was highly cOlTelated with bridge length (r> 0.95) it was not included in the models. We used an 0: :s;; 0.05 to determine statistical significance for all tests. Data arc presented as the mean ± SD throughout the results.

'Ve used program PRESENCE (MacKenzie et al., 2002) to estimate the probability of detecting bats (p) under a bridge and bridge occupancy ('1') for the 2003 sampling period. 'Ve used estimates of p to determine the minimum number of times a bridge needs to be inspected to determine whether it is occupied. However, because the bridges \vere not chosen randomly, p and '¥ are biased. Models were run on all bridges inspected once in 2002 and 2::2 times in 2003. We included bridge use in 2002 as a covariate to test whether previous occupation of a bridge ,,·laS an important variable in detection probabilities and occupancy in 2003. 'Ve compared models using Akaike's Information Criterion

2008 BENNETT ET AL.: BAT RooSTS 391

corrected for small sample sizes (Alec; Burnham and Anderson, 2002). Models with flj > 2 were not considered to have strong support. 'Ve estimated the minimum number of surveys needed to detect bats at a bridge using the following equation (MacKenzie and Royle, 2005):

p * = 1 - (1 _ p)K

where p* = the probability of finding bats at a bridge at least once, p = the probability of detection, and K = the number of surveys.

RESULTS

BRIDGE ROOSTS OF RAFINESQUE'S BIG-EARED BATS

'Ve sUlveyed 1129 bridges in 2002 and conducted 443 surveys of 235 bridges in 2003. Overall, we surveyed 7.1 % of the slab bridges, 17.4% ofthe MB btidges and 83.4% of the TB bridges in the state, representing 17,9% of all bridges spanning permanent water bodies. '~le found Rafinesque's big-eared bats beneath 38 bridges (3.4%) in 2002 and 54 blidges (22.9%) in 2003. ·Many bridges \vere used in both years (see below); the total number of occupied bridges was 73. Colonies and solitary bats were sometimes found under the same bridge, but were always spatially separated. In 2002, we observed 196 big-eared bats in colonies (n = 13 bridges) and 49 solitary bats (n = 33 bridges). Colonies observed in 2002 ranged in size from 2~53 bats (median = 12 bats). These numbers are conservative as it often was difficult to get an exact Count of bats, particularly when neonates still clung to their mother. In 2003, colonies and solitaty bats roosted beneath 24 and 47 bridges, respectively. The number of bats (range = 2~31 bats) in colonies fluctuated throughout the 2003 monitoring period; the median number of bats in a group was eight. ''''''e found multiple roosts of solitary bats beneath nine bridges; one large bridge had five separate solitary big-cared bats roosting beneath it at one time.

In 2003, we were unable to inspect eight bridges occupied by solitary bats the previous year due to logistical and time constraints. Vve located Rafinesque's big-eared bats under an additional 35 bridges in 2003. Twentr-six of these bridges had been inspected in 2002 and did not have any day roosting big-eared bats, but 15 bridges (57.7%) had feces in 2002.

Big-eared bats primarily roosted over the dq' banks on either end of a bridge near the abutments; we found 4 of 108 (3.7%) solitary bats roosting in the middle section of bridges where the \\'<lterbed was dry, and three of 37 (8.2%) colonies over both water and dlY bank under bridges where there was little dry substrate present. Rafinesque's big-eared bats roosted between support beams in the moderately open areas of a bridge; they were never found in small expansion joints. Bats occasionally flew to adjacent sections of the bridg~ during surveys; however, bats were only observed leaving bl;dges twice during the study. Rafinesque's big-eared bats did not leave fecal stains on the bridge walls. Fecal pellets were most often observed as individual pieces on the concrete walls of the bridges; occasionally we found guano in larger quantities on the ground.

V\Te observed other bat species beneath 45 bridges dUling the statewide survey: eastern pipistreHe (Pe~imyolis subjlnvus, n = 26); big brown bat (EptesicllS juscus, n = 10); southeastem m}'otis (.Myolis auslroripmius; n = 1); Brazilian free-tailed bat (Tadarida brasiliensis, 11 = 1); and unidentified J\f.yotis species (n = 7). Solitary big-eared bats roosted under blidges with birds (n = 7) and other bat species (n = 3). but did not roost under bridges where domestic animals were found. When Rafinesque's big-eared bats used the same bridge as other species, they usually used separate sections of a bridge. However, we

392 THE AMERICAN MIDLAND NATURALIST 160(2)

TABLE I.-Numbers and percentages (in parentheses) of bridges surveyed in South Carolina occupied by Rafincsque's big-eared bats and other species of bats, i'.Iay-Aug. 2002. n = number of bridges examined

Bridge \ariable n Rafinesque's big-eared bats Other bat spp.

T)pe Slab 28'1 o (0.0) o (0.0) !>'Iulti-Beam 281 6 (2.1) 15 (5.3) T-Beam 564 32 (5.7) 28 (5.0)

r ... faterial Concrete 1015 38 (3.9) 41 (4.0) Steel 87 o (0.0) 2 (2.3) Timber 27 o (0.0) o (0.0)

Disturbance 0 318 3 (0.9) .]4 (4.4) .1 381 12 (3.2) 19 (5.0) 2 275 18 (6.6) 9 (3.3) 3 155 5 (3.2) I (0.7)

Region Blue Ridge 32 o (0.0) 2 (6.3) Piedmont 381 2 (0.5) 23 (6.0) Upper Coastal Plain 502 23 (4.6) 16 (3.2) Lower Coastal Plain 214 13 (6.1) 2 (0.9)

located one maternity colony of big-eared bats (n = 37 bats) roosting next to a maternity group of big brown bats (n = 6 bats) on a single occasion in 2003. '''''e regularly found both species beneath this bridge.

USE OF BRIDGES BY PH\SfOGRAPHIC REGION

Bridges used by Rafinesque's big-eared bats in 2002 were not distributed evenly across the state (Fig. 1, Table 1). Although the majority of big-eared bat roosts (94.7%) were beneath bridges in the Upper and Lower Coastal Plains. a sman percentage (5.3%) were located in the Piedmont. No bridges in the Blue Ridge Mountains were used by bats. vVe found a significant association between physiographic region and presence of Rafinesque's big­eared bats (x' ~ 22.7, df ~ 3, P '" 0.001, n ~ 1129) in 2002. However, we did not find significant differences in the presence of bats between Upper and Lower Coastal .Plain regions in 2002 (x.2 ~ 0.7, df ~ 1, P ~ 0.411, n ~ 716) or 2003 (x' ~ 1.9, df~ 1, P ~ 0.169, n = 235). We established new county records for Rafinesque's big-eared bats in Allendale. Bamberg, Barnwell. Chesterfield, Dillon. Horr)" Marion, Orangeburg. Sumter and \Villiarnsburg counties.

In the Coastal Plain, many of the bridge roost sites were grouped within the same ,vatershed (Fig. 1). The South Fork of the Edisto River and its tributaries, which cross both the Upper and Lower Coastal Plain in the western part of South Carolina. contained the highest concentration of occupied bridges (23.3%). In addition, we found 12.3% of the bridge roosts along the Great Pee Dee River in the eastern portion of the state.

BRIDGE ATTRIBUTES ASSOQATED WITH ROOST SELECTION

In 2002. the presence ofRafinesque's big-eared bats was associated with bridge type (%2 =

28.6, df ~ 2, P '" 0.001, n ~ 1129), disturbance (x.2 ~ 14.8, df ~ 3, P ~ 0.002, n ~ 1129), bridge length (x.2 ~ 17.35, df ~ 1, P '" 0.0001, n ~ 1120), width (x.2 ~ 12.56, df ~ 1, P '"

2008

2000

1500

Me o 1000

g .~ (/J 500

BENNETI ET AL.: BAT ROOSTS

_ Occupied

~ Unoccupied

Length

iiffl

Width

2002

n Area Length

iIi@

Width

2003

393

Mea

FIG. 2.-1Jean length, -width and area of occupied and unoccupied bridges by Rafinesque's big-eared bats in South Carolina May-Aug. 2002 and 2003. Error bars represent 1 SD

0.0001, n = H20) and area ()(,2 = 23.20, df = 1, P ,-; 0.0001, n = ll20). We only located Rafinesque's big-eared bats beneath TB and IvlB bridges; no hats were observed under slab bridges (Table I), 1Nhen slab bridges were excluded from the analyses, similar results were obtained: presence of big-eared bats was associated with bridge type (X2 = 6.2, df = 1, P =

0.013, n = 845), disturbance ()(,2 = 9.8, df = 3, P = 0.02, n = 845), length (X: = 9.67, df = 1, P = 0.002, n = 842), width ()(,2 = 9.22, df = 1, P = 0.002, n = 842) and area (x' = 15.18, df = I, P < 0.002, n = 842). The presence of other l;mt species was not associated with roosting big-eared bats (P = 0.120). Occupied bridges had a median disturbance level of 2. In general, 'ive found bats under large girder bridges that were 36.4% longer, 13.7% wider, and covered 37.2% more area than bridges not occupied (Fig. 2). All occupied bridges were concrete.

Results of the logistic regression analysis for bridge selection in 2002 indicated that physiographic region ()(,2 = 24.5, df = 2, P s 0.0001), bridge type (X2 = 20.1, df = 1, P ,-; 0.0001) and bridge width ()(,2 = 5.2, df = 1, P = 0.023) were the best predictors of big-eared bat presence. The overall regression equation , ... as significant (X2 = 57.8, df = 3, P :-:; 0.001) and the model did not deviate from a logistic fit (X2 = 6.1, df = 8, P = 0.637). Interactions between vadables were not ·significant and were removed from the final model. Although bridge length and disturbance differed significantly between occupied and unoccupied bridges in the univariate analyses, they did not enter the model indicating no influence on selection of bridge roosts when the other variables were controUed. 'Ve obtained similar results when we excluded slab bridges from the analyses. Presence of big-eared bats was influenced by physiographic region ()(,' = 28.8, df = 1, P < 0.0001), bridge type ()(,' = 4.2, df = 1, P = 0.04) and bridge "idth ()(,2 = 5.1, df = 1, P = 0.02). The overall model was significant ()(,' = 42.7, df = 3, P < 0.0001) and did not d"iate from a logistic fit ()(,2 = 12.8, df = 8, P = 0.117).

In 2003, presence of bats was significantly associated ,."ith bridge type and disturbance level. As in 2002, we found Rafinesque's big-eared bats only beneath TB and MB bridges

394 THE AMERICAN MIDLAND NATURALIST 160(2)

TABLE 2.-Number and percentage (in parentheses) of highway bridges in the Upper and Lower Coastal Plains of South Carolina occupied by Rafinesquc's big-eared bats, ~-fa}'-Aug. 2003. n = number of bridges examined

Bridge variable n Rafinesque's big-eared bats

T)pe Slab 11 o (0.0) Multi-Beam 38 7 (18.4) T-Beam 187 47 (25.0)

Disturbance 0 130 29 (22.3) I 86 24 (27.9) 2 13 I (7.7) 3 7 o (0.0)

Region Upper Coastal Plain 120 32 (26.7) Lower Coastal Plain 116 22 (19.0)

(Table 2). There was a significant association between structure type and occupancy (X; = 6.8, df = 2, P = 0.034, n = 235). Although TB btidges were >6 times more likely to be used than MB bridges, there was no statistical difference in occupancy between MB and TB in 2003 ex" ~ 0.8, df ~ 1, P ~ 0.357, n ~ 224). Most big-eared bats used bridges with a disturbance level of 0 or 1 in 2003, and there was a significant association between disturbance level and bat presence (P = 0.001, n = 235).

Bridges occupied by big-cared bats in 2003 were longer ("l = 8.7, df = 1, P = 0.003, n =

227) and had greater area ('I: ~ 11.9, df ~ 1, P ~ 0.0006, n ~ 227) than unoccupied bridges (Fig. 2). \,Vldth did not differ significantly between occupied and unoccupied bridges (l =

3.6, df = 1, P = 0.06). Bridge type was the only variable that entered into the logistic regression model (X2 ~ 5.9, df ~ 1, P ~ 0.02).

RoosI' FIDELITY AND PROBABIUTYOF DETECTION

'''Ie visited occupied bridges an average of 3.06 times (range 1-5) and unoccupied bridges 2.0 times (range 1-5) during the 2003 SUlyey. For bridges that were occupied at least once and inspected more than once, the frequency of bridge use ranged from 33% to 100% (mean = 65.9% ± 24.7%). For all bridges (occupied and unoccupied) inspected more than once, frequency of use was 24.7 ± 35.8%. Of the 30 bridges occupied in 2002 and surveyed in 2003,19 (63.3%) were occupied both years.

Bridge use in 2002 affected both detection probability and occupancy in 2003 (Table 3). Probability of detecting Rafinesque's big-eared bats under bridges in 2003 that were not

TABLE 3.-l\'fodel selection results for probability of detection and occupancy by Rafinesque's big­eared bats under bridges in the Upper and Lower Coastal Plains of South Carolina, Mar-Aug. 2003. Occupancy ('¥) and probabilit}'of detection (p) were modeled with or without consideration of whether the bridge w-as occupied in 2002 (Occ02). (.) indicates that Occ02 was held constant

Model K Alec AAICc

'I'(Occ02) P(Occ02) 4 303.10 0 '1'(.) p(Occ02) 3 307.09 3.99 'I'(Occ02) P(.) 3 308.54 5.44 '1'(.) P(.) 2 320.54 17.44

2008 BENNEIT ET At.: BAT ROOSTS

i

30 ~ Number of bats -*- Percent occupied

2'

20

~ 16 C

~ 10

5

i:' ~ c c • , , " " ~ ~

~ ~ ~ 0 N N .

c . ~ i:'

, c • ~ ,

" " " ~ ,..

t:l :<l

c c 'S 'S , , ~ ~

~ ~

1'< '" N :: . ": .

~ c

c , "3 , , ~ ~ ~ ~ 0 ~

12 ~ "

395

80

70

60 '0

50 .! " u u

40 0 -c .. 30 !!

~ 20

10

0

" "3 g> ~ ~

" ~ ~ N .. .. " ~ ~ ~

N '" !'l ~ N

FIG. 3.-l\ofean number of Rafinesque's big-eared bats under bridges and the percent of bridges occupied in the Coastal Plain of South Carolina from late May through early Aug., 2003. Error bars represent 1 SD

llsed in 2002 was 0.457 and occupancy was 0.385. In contrast, p for bridges that had bats in 2002 was 0.730 and occupancy was 0.727. Thus, if bats were not observed under a bridge in 2002, there was a 91 % chance of detecting bats under the same bridge in 2003 if they were inspected four times and a 95% chance of detecting them jf they were inspected five times. By contrast, if bats were observed under a bridge in 2002. there was a 93% chance of detecting them in 2003 if the bridge was inspected only twice and a 98% chance of detecting them if it was inspected three times.

SEASONAL BRIDGE USE

The highest mean number of bats per occupied bridge (14.0 ± 11.8 bats) was observed in mid:Jun. (Fig. 3). However, we recorded ~he highest percent of occupied bridges (69.2%) in carly JuI. In general, although the mean number of bats per occupied bridge was similar throughout the 2003 survcy period, we found more occupied btidges duting the latter part of the survey.

DISCUSSION

Our study represents the most extensive regional bridge survey conducted for Rafinesquc's big~eared bats. Although only a small proportion of the state's bridges were occupied, they were occupied by both maternity colonies and solitary bats, and were used over multiple years. However, use of bridges was not distributed randomly with respect to region. bridge type or size. Bats selected large girder bridges. primarily in the Upper and Lower Coastal Plains. Results from the logistic regression analysis indicated the importance of physiographic region and bridge type to the bat's distribution and selection of roosts.

396 THE AMERICAN MIDLAND NATURALIST 160(2)

The distribution ofRafinesque's big-eared bats under bridges in South Carolina "''as similar to histOIical records for this species (Menzel et aL, 2003). Although new occurrence records were obtained for 10 counties, they were all within the bat's previously described range. The majoIil}' of bridges used as big-eared bat day roosts were located in the Upper and LO\vcr Coastal Plains. However, two bridge roosts were located in the Piedmont. Both bridges contained solitary bats and were located <30 km from the Upper Coastal Plain within bottomland deciduous forests, the bat's primary roosting habita.t in the Coastal Plain (Clark, 1990; Lance el ai., 2001; Trousdale and Beckett, 2005). Thus, it appears that Rafinesque's big­eared bats arc largely absent from the Piedmont of South Carolina, but use localized tracts of bottomland hardwood forest that are contiguous with those of the Coastal Plain.

It is unlikely that we failed to locate a significant popUlation ofRafinesque's big-eared bats in the Piedmont. Although bottomland hardwood forests and large cavity forming trees are present in this physiographic region, they are uncommon. However, suitable bridges for roosting arc plentiful (275 TB bridges and 1188 MB bridges). Thus, if bats were present they would be expected to be beneath bridges. Although we determined that at least two to five surveys are necessal)' to determine whether a bridge is being used, we used the same sampling method (i.e., one survey per bridge) in the Coastal Plains and located bats under 36 bridges ..

The lack of occupied bridges in the Blue Ridge Mountains was suq)fising. Rafinesque's big-eared bats were captured and obsen'ed in this region during the time of the sun'ey by two of the authors (SCL, 1\'ISB) thus, we expected to find them using bridges. However, the Blue Ridge region has fewer suitable bridges (only 25 TB bridges and 5 h-IB bridges) than the Coastal Plain (376 TB bridges and 332 MB bridges). Thus, if bats used bridges at the same rate in the Blue Ridge as in the Coastal Plain, they only would be expected to occupy one bridge. Moreover, use of artificial roosts appears to be rare in the northern portion of the range (Jones, 1977). Although a maternity colony was observed under one bridge in Kentucky (James Kiser, in litt.), no Rafinesquc's big-eared bats were found under 232 bridges in southern IHinois (Feldhamer el at., 2003). Thus, bridge use may be rare in the northern portion of the range, perhaps because natural roost sites such as rock houses, caves, tree cavities and abandoned mines are still abundant. Ivloreover, because a large percentage (>60%) of the Blue Ridge ~ .. Iountains is protected through state conservation programs or is in federal ownership, natural roost sites may be more available than in other parts of South Carolina.

1vVe found a strong relationship between presence of Rafinesque's big-eared bats and bridge type. Results of the logistic regression analysis indicated the odds of locating big­eared bats were highly dependent upon bridge structure. Although we only examined a sman proportion of the slab bridges in the state, we found no evidence of use of these bridges by Rafinesque's big-eared bats or other species. Our observations are consistent with the pattelu of bridge use by Rafinesque's big-eared bats in other states. In Louisiana, approximately 97% ofRafinesque's big-eared bat observations were from girder type bridges (Lance et at., 2001) and in the Coastal Plain of North Carolina 100% of the Rafinesque's big­eared bat observations from girder bridges (McDonnell, 2001).

Although many of our findings were similar to previous studies, we found some unique patterns of bridge selection in this study. Rafinesque's big-eared bats showed a higher rate of occupancy in TB bridges (9.1%, n. = 31 of 341 TB bridges; Upper and Lower Coastal Plains only) than :MB bridges (3.2%, n = 5 of 155 bridges; Upper and Lower Coastal Plains only) in South Carolina. However, in the Coastal Plains of North Carolina, the rate of occupancy was higher beneath MB bridges (16.6%, n = 29 of 259 MB bridges) than TB bridges (9.0%, n =

2008 BENNETT ET AL.: BAT ROOSTS 397

6 of67 TB bridges; McDonnell, 2001). In addition, big·eared bats used a small number of timber ME bridges in North Carolina (5.0%, n = 6 of 120 bridges; IvIcDonnell, 2001); no timber bridges were occupied in either year of the South Carolina sun'er (0 of27 bridges), Differences in the use of MB versus TB bridges rna)' be related to other bridge variables such as surrounding habitat and roost microclimate, which are significant factors in the selection of roosts by Rafinesquc's big-eared bats (Clark, 1990; Lacki, 2000; Lance et at., 2001). Measurement of these extrinsic variables may help to resolve the difference in bridge use patterns within the Coastal Plain.

Occupancy of larger bridges also was unique to Rafinesque's big-eared bats in South Carolina. Bridge use in Louisiana and North Carolina was not related to length or width (Lance et al., 2001; McDonnell, 2001), but in South CaroHna, occupied bIidges were longer, wider and covered a larger area than bridges not occupied. Bridge size is a significant factor in night roost selection for many bats in western North America (Perimeter, 1996; Adam and Hayes, 2000). Larger bridges maintain higher nighttime temperatures thus, provide a better microclimate than smaller bridges. Higher nighttime temperatures may be a particularly significant factor for maternity colonies during the lactation period when females leave their young to forage (e.g., Chruszcz and Barclay, 2002). Larger bridges also may provide a greater diversity of microclimates, allowing bats to choose among them as environmental conditions change, as wen as providing greater protection from predators (Ferrara and Leberg, 2005a).

Disturbance 'was a significant variable in roost selection by bats, but was inconsistent between years, In the 2002 statewide survey, bridges with a disturbance rating of2 were more likely to be occupied than any other bridges. In 2003, most occupied bridges had a disturbance rating of O. The difference between years was probably due to variation among observers. The apparent occupation of bridges with a high level of disturbance in 2002 is in disagreement with other studies (Lacki, 1998, 2000; Lance et at., 2001), likely because disturbance caused by traffic on bridge surfaces and the disturbance levels underneath a bridge were not separated in our study. Often, bridges with heavy traffic levels had little disruption underneath the stmcture; this may account for the occupation of bridges with seemingly elevated disturbance in 2002.

Rafinesque's big-eared bats exhibited high short-term fidelity to bridges in the Coastal Plains of South Carolina. For structures occupied at least once, the frequency of use was 65.9%. Although this estimation of bridge roost fidelity may be somewhat biased because bridges were inspected more often if bats initially were present, the frequency of bridge use was similar to other studies. Lance et at. (2001) reported that female big-eared bats primarily used bridges, but that the proportion of days spent at a bridge roost varied from 20% to 100%. The remaining roost days were spent in trees (Nyssa spp.). Ferrara and Leberg (2005b) also found high short-term fidelity to bridge roosts by tagged individuals. 'Ve found that the probability of finding bats under a bridge and bridge occupancy in 2003 were strongly associated 'ivith presence of bats under the bridge in 2002. This indicates there .. vas strong year-to-year fidelity to bridges. High roost fidelity is directly related to the permanency ofa structure and inversely related to roost availability (Lewis, 1995). Bridges are permanent, available and abundant in South Carolina, so the high fidelity of Rafinesque's big-cared bats to bridges is not surprising,

Our results suggest that large-scale bridge surveys may be a good method for detelmining the distribution of Rafinesque's big-eared bats across a region, as well as for locating local colonies and individuals. While surveys can be conducted throughout the summer, JuI. represents the time when the most bridges were occupied. Further, young Rafinesque's big-

398 THE AMERICAN MIDlAND NATURALIST 160(2)

eared bats are independent by early Jul. (Jones and Suttkus, 1975) thus, disturbance of maternity colonies should be reduced during this time. Although surveying bridges once is sufficient for determining large-scale distribution patterns, multiple surveys are necessary to determine whether a particular bridge is occupied by bats. Our estimates of p and '¥ in 2003 were biased because we did not select bridges at random. Thus, our estimates of the number of times that bridges should be inspected represent the lowest end of the range. However, based on our estimates, a bridge should be inspected at least four or five times during the summer if there is no indication of prior use and at least two times if the bridge was used in the prior year to determine whether it is currently being used. Ferrara and Leberg (2005b) suggested that bridges be inspected ~3 times to ensure that a known roost is not being occupied. However, we suggest that further studies be conducted using repeat ,~sits of randomly selected bridges to obtain unbiased estimates of the minimum number of sun'eys necessary to determine whether a bridge is being occupied by Rafinesque's big-eared bats.

J

Admowle.dgmenls.-Financial and logistical support for this research was provided by the U.S. Fish and "\Vildlife Service's Candidate Conservation program, the South Carolina Department of Natural Resources and the USDA Forest Service Southern Research. Station. We thank A. Trousdale and two anonymous revicwcrs for valuable comments and 1\1. Collingwood, C. Dachelet, D. Eggert A. Eizerman, J. Schwenter and A. Sjollema for field assistance. Thanks to W. Bridges for help with statistical analyses.

LITERATURE CITED

ADAM, M. D. AND J. P. HAYES. 2000. Use of bridges as night roosts by bats in the Oregon coast range.] Mammal., 81:402-407.

BARBOUR, R. AND W. DAVIS. 1969. Bats of America. University Press of Kentucky, Lexington, Kentucky. 286 p.

BENNElT, F. M. 200'1. A survey of the statewide distribution and variation in metal concentrations from the hair of Rafinesque's big-eared bats (Corynorhinus rajinesquif) in South Carolina. Thesis. Clemson University, Clemson, South Carolina, USA. 121 p.

BUNCH, M.j.,]. SORROW AND A. S. DYE. 1998. Rafinesque's big·eared bat surveys and prelisting recovery. Unpublished Final Report for the U.S. Fish and Wildlife Service by the South Carolina Department of Natural Resources, 82 p.

BURNHAM, K P. AND D. R. ANDERSON. 2002. Model selection and multi·model inference. Springer, New York. 488 p.

CHRUSZCZ, B. J. A,'W R. M. R. BARCLAY. 2002. Thermoregulatory ecology of a solitary bat, Myolis rootis, roosting in rock crevices. Fund. Ecol., 16:18-26.

Cl.ARK,11. K 1990. Roosting ecology of the eastern big·eared bat (Pleco/Us rafinesquit) , in North Carolina. Thesis, North Carolina State University, Raleigh, USA. III p.

CONNER, R. C. 1993. Forest Statistics for South Carolina. USDA Forest Service, Southeastern Forest Experiment Station Resource Bulletin SE-141. 52 p.

ERICKSON, G. A. 2002. Bat and Bridges Technical Bulletin (Hitch Hikers Guide to Bat Roosts). California Dept. Transportation, Sacramento, CA. 2002. 76 p.

FEI-DHA. ... IER, G. A., T. C. CARffiR, A. T. MORZILLO AI'l"D E. H. NICHOLSON. 2003. Use of bridges as day roosts by bats in southern Illinois. '])-ans. Ill. State Acad. Sci., 96:107-112.

FELTS.]. W. AND 'W. D. WEBSffiR. 2003. The use of bddges as daytime roosts by bats in southeastern North Carolina.] No. Car. Acad. Sci., 119:172-178.

FERRARA, F. j. AND P. L. LEBERG. 2005a. Characteristics of positions selected by dar-roosting bats under bridges in Louisiana. J Mammal., 86:729-735.

--- AND ---. 2005b. Influence of investigator disturbance and temporal variation on sun'eys of bats roosting under bridges. Wildl. Soc. Bull., 33:1113-1122.

FREEMAN, G. H. AND]. H. HALTON. 1951. Note on an exact treatment of contingency, goodness of fit and other problems of significance. Biome{rika, 38:141-149.

2008 BENNETf HAL.: BAT ROOSTS 399

GoODING, G, AND J. R. LANGFORD. 2004. Characteristics of tree roosts of Rafinesque's hig-eared bat and southeastern bat in northeastern Louisiana. Southwest. Nal., 49:61-67.

HANDLEY, C. O.,JR. 1959, A revision of American bats of the genera Eud.enna and Plecalus. Prac. U.S. Nat.

Mus" 110:95-246. HARVEY, i\'f. J., J. S. ALTENBAcn AND T. L. BEST. 1999. Bats of the United States, Arkansas Game & Fish

Commission. 64 p. HOSMER, D. W .• JR. AND S. LEMESHOW, 2000. Applied Logistic Regression, second edition. John \\1Jleyand

Sons, New York, New York. 392 p. HURST, T. E. AND i\1. J. LAcKI. 1999. Roost selection, population size and habitat use by a colony of

Rafinesque's big-eared bats (CorynQrhillus rafinesquit). Amer. ~Midl. Nat., 142:363-371. JONES, c, }977. Pleco/us raj{nesquii. Mammal. Species, 69:1-4. ---AND R. D. SUTIKUS. 1975. Notes on the natural history of Pl.ec()tus rajinesqltii. Occ. Papers jUus. ZlJol.,

Louisiana Staie Univ., 4'7:1-13. KEELEY, B. "\V. A.,~D M. D. TUTTLE. 1999. Bats in American bridges. Resource Publication No, 4. Bat

Conservation International, Austin, Texas. LACKJ, M.]. 1998. Monitoring of Virginia big-eared bats and Rafinesque's big-eared bats at Hood Branch

Rock Shelter. Natural BIidge State Park Nature Presen'e. Powell County, Kentucky. Kentucky State Nature Presen'e Commission, Frankfort, Kentucky.

---. 2000. Effect of trail users at a maternity roost of Rafinesque's big-eared bat:>.}. Cave and Karst Stud., 62:163-168.

LANCE, R. F., B. T. HARDCASTLE, A. TAllEY AND P. L. LEBERG. 2001. Day·roost selection by Rafinesque's big­eared bats (C01),norhinlls mjinesquit) in Louisiana forests.}. .I.\Jammal., 82:166-172.

LF.\\1S, S. E. 1995. Roost fidelity of bats: A review. J j't,lammal., 76:481-196. ~1ACKENZIE, D. I., j. D. NICHOI..S, G. B. LACHMAN, S. DROEGE,]. A RmLE AND C. A. LANGTIMM. 2002.

Estimating site occupancy rates when detection probabilities are less than one. Ecoi.ogy,

83:2248-2255. --- AND A. RmLE. 2005. Designing occupancy studies: general advice and allocating survey effort. J

Appl. Ewl., 42:1105-1114. MARTIN, C. 0., W. A. MITCHELL AND M. S. WOLTERS. 2002. Eastern cave- and crevice-dwelling bats

potentially impacted by U.S. Anny Corp of Engineers resen'oir operations. EMRRP Technical Notes Collection (ERDC TN EMRRP-SI-24). U.S. Army Engineer Research and Development Center, Vicksburg, ~Hssissippi, USA

McDoNNELL,j. M. 2001. Use of bridges as day roosts by bats in the North Carolina Coastal Plain. Thesis, North Carolina State University: Raleigh, North Carolina, USA. '74 p.

MENZEL,j. M., M. A. MENZEL, W. 1-1. FORD,]. W. EDWARDS, S. R. SHEFFlELD,J. C. KILGO At'lD M. S. BUNCH. 2003. The distribution of the bats of South Carolina. Southeast. Nat., 2:121-152,

MlROWSRY, K. 1\J" P. A. HORNER, R. W. MAXEY AND S. A. S)'lllH. 200'1. Dist.ributional records and roosts of southeastern m)'otis and Rafinesque's big-eared bat in eastern Texas. Southwest. Nat.,

49:29'1-298. PERU.rETER, S. I. 1996. Bats and bridges: patterns of night roosting activity in the Willamette National

Forest, p. 132-150. In: R. M. R. Barclay and R. M. Brigham (eds.). Proceedings of the bats and forests symposium, Oct. 19-21, 1995. British Columbia Ministry of Forests Research Branch, Victoria. British Columbia. 292 p.

SAS INSlTTUfE. 2002. SAS user's guide. Version 9.0. SAS Institute. Cary, North Carolina, USA. TROUSDALE, A. W. AND D. C. BECREIT. 2002. Bats (I\fammalia: Chiroptera) recorded from mist-net and

bridge stln'eys in southern Mississippi.}. Miss. Acad. Sci., 4'7:183-189. ---AND ---. 2004. Seasonal use of bridges by Rafinesque's big-eared bat, CorynorMnus mjinesquii,

in southern IVIississippi. Southeast. Nat" 3:103-112. -'-- AND ---. 2005. Ch~racteristics of tree roosts of Rafinesque's big-e<tred bat, COl),norhinlls

rafinesquii, in southeastem Mississippi. Amer. Afidf. Nat., 154:442-449.

SUB!>I1TTED 21 DECEMBER 2007 ACCEPTED 27 MARCH 2008